1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices, and more particularly to an improved method of manufacturing semiconductor devices in which impurity conductive layers are formed on the surface of a semiconductor substrate.
2. Description of the Prior Art
In recent years, the constituent components of semiconductor integrated circuits such as DRAMs (dynamic random access memory) have been significantly reduced in size to meet higher integration requirements. In general, DRAMs use memory cells each of which is constituted on the basis of one-transistor/one-capacitor. Thus, the further reduction in the size of the memory cell is very important to achieve higher-integration. When reducing the size of the memory cell, the following must be taken into consideration. Specifically, when a cell capacitor, which is a constituent component of the memory cell, is reduced in size, the amount of electric charge stored therein also decreases. This causes the cell capacitor to operate erroneously in response to noise. Namely, a S/N ratio (signal-to-noise ratio) having a sufficient read-out margin cannot be secured. Thus, the cell capacitor must store a sufficient amount of electric charge to avoid such an erroneous operation. A technique is known in which the thickness of the insulating film of the cell capacitor is reduced to secure a prescribed amount of electric charge. However, this technique has reached the limit in reliability with respect to the electric field strength. Instead of this technique, a trench-capacitor technique in which an effective surface area of the cell capacitor can be obtained to secure a prescribed amount of electric charge, advantageously is employed. Specifically, as shown in FIG. 6a, trench capacitor cells are constituted by plural trenches 65 formed on the surface of a P-type silicon substrate 61, a capacitor oxide film 62, an N-type impurity-doped polycrystalline silicon layer 63 and an N-type impurity diffusion layer 64.
In FIG. 6a, the impurity diffusion layer 64 having a high concentration is formed outside the trenches 65 in the substrate 61. In this structure, depletion layers often do not develop on both sides of the trenches 65. Thus, the capacitors also may be operated at voltages of 1/2 Vcc in positive and negative polarities. As a result, the breakdown voltage of the capacitor insulating films is reduced. In this case, a technique to diffuse impurities very precisely into narrow and deep trenches is very important.
In this doping technique, ion implantation is widely used to form a diffusion layer because very precise amounts of impurity can be introduced. However, when ion implantation is applied to a trench structure, the following problem arises. Specifically, impurity ion implantation is intercepted by the vertical sidewalls of trenches 65 as shown in FIG. 6a. This is because the direction of impurity ion implantation is constant. Thus, regions where impurities are not introduced are developed. To solve this problem, several techniques can be considered. For example, the incident angle of implantation may be changed, or the silicon substrate may be rotated. However, both techniques are very difficult to apply to a trench structure which is expected to be used in the future of higher integration. Namely, the future trench will become even narrower and deeper.
In place of ion implantation, a technique in which impurity diffusion from a solid-phase diffusion source is utilized, can be considered. Specifically, as shown in FIG. 6b, an impurity-doped polycrystalline silicon film 63 or a silicon oxide film is formed within the trench 65 as a solid-phase diffusion source. Thereafter, impurities are introduced from the solid-phase diffusion source into the portions outside the trench 65 by use of thermal diffusion. In this case, a homogeneous film having a prescribed impurity concentration must be uniformly formed on the inner wall of the trenches 45 as an impurity diffusion source. However, trenches are becoming narrower and deeper with the progress of integration. Thus, formation of a uniform impurity diffusion source will become more difficult. Several techniques have been considered to meet such requirements.
For example, in the case of the CVD (chemical vapor deposition) technique, an impurity diffusion source cannot be uniformly formed inside the trench. Specifically, the film of the impurity diffusion source becomes thinner toward the bottom portion of the trench. This thinned film causes an insufficient amount of impurity diffusion. As a result, the concentrations of the impurity diffusion layer 64 become nonuniform with respect to the upper and bottom portions of the trench 65. In another technique, a solution containing impurities is painted on the inner sidewalls and bottom of the trench. The solution drys so as to form an impurity diffusion source. This technique has disadvantages such that bubbles in the solution can remain within the trench. Further, when the trench becomes narrower and deeper, it is more difficult to completely introduce the solution into the entire portions within a large number of trenches (more than one million trenches per chip, for example).
As described above, in the conventional techniques, it is very difficult to introduce precise amounts of impurity into a semiconductor substrate having a significant uneveness on the surface structure thereof.
Another technique can be considered to solve this problem. In this technique, impurity elements or compounds containing impurity elements are obtained by a thermal decomposition reaction from vapor-state impurity elements or compounds containing impurity elements. Then, these materials are adsorbed to the inside of trenches and serve as an impurity diffusion source. This technique can form an impurity diffusion layer having a uniform concentration on the entire portions within the trenches. However, a natural oxide film is formed spontaneously on the surface of a semiconductor substrate. Moreover, such a natural film grows rapidly in the air, and the elimination thereof is difficult. Further, the natural oxide film obstructs the adsorption of conductive-layer forming elements or compounds containing one of these elements. As a result, impurities cannot be sufficiently diffused into the substrate.